The present invention relates to cation complexes. More particularly it relates to cation complexes that comprise a cation guest and a polydentate polymeric host that contains cyclic ether units joined together through either tetrahydrofuran 2,5- or tetrahydropyran 2,6-diyl linkages (or combinations thereof) to provide a specified array of multiple hetero atoms (described more fully hereinafter). The complexes of the invention result from ion-dipole interaction between the cation and the array of heteroatoms.
A variety of cation complexes have been reported in the literature. They comprise a cationic guest and a complexing host. The host typically contains a plurality of heteroatoms arranged in an array that makes them available for multi-dentate coordination with an appropriate cation.
Examples of known host compounds are poly(ethylene oxide), crown ethers (i.e., macrocyclic oligomers of ethylene oxide), cryptands (i.e., macrocyclic compounds that contain ether oxygen and amino nitrogen in a special array), and naturally occurring ionophores. The characteristics of these hosts and the manner in which they form complexes with cations have been reported. Thus, for example, see Poly(ethylene oxide), F. E. Bailey, Jr. and J. V. Koleske, pp. 94-102, Academic Press, 1976; C. J. Pedersen, J. Amer. Chem. Soc., 89, p. 7017 (1967); U.S. Pat. No. 3,562,295; C. J. Pedersen and H. K. Frensdorff, Angewandte Chemie, International Edition 11, p. 16 (1972); J. M. Lehn, "Structure and Bonding" 16, Chapter 1, Springer-Verlag (New York), 1973; J. M. Lehn, Accounts of Chem Research 11, pp 49-57 (1978); D. J. Cram and J. M. Cram, Accounts of Chem. Research 11, pp 8-14 (1978); J. J. Christensen, D. J. Eatough and R. M. Izatt, Chem. Revs. 74, p. 351 (1974); and U.S. Pat. No. 3,966,766.
Each of these hosts exhibit characteristics that limits it usefulness. For example, while poly(ethylene oxide) forms complexes with certain cations, the complexes are relatively unstable when compared with those formed between cations and either crown ethers or cryptands. This is due to the fact that, in solutions, poly(ethylene oxide) is capable of assuming numerous chain conformations, many of which are not effective in forming cation complexes.
Crown ethers and cryptands, on the other hand, can form stable complex compositions with certain cations. The cyclic structure of these compounds imposes relatively severe conformational constraints upon them thereby rendering them effective cation complexing agents for only certain cations. The conformations adopted during complexation are such as to provide cavities (i.e., holes in the molecule) bounded by electron-donor heteroatoms, thus providing a suitable chemical environment for ion-dipole coordination (i.e., complexing) with cations of suitable size. The cavities have fixed sizes that act as a strong determinant in controlling the stability of the complex formed between a given macrocycle and a given cation. Thus, the size of the cavity and the diameter of the cation must be relatively closely matched in order to provide stable complex compositions. These compounds are not, therefore, capable of forming stable complex compositions with a wide variety of cations of differing sizes.
The ionophores are typically either macrocyclic compounds per se (e.g., valinomycin) or compounds that have functional end groups that favor the formation of macrocycles via strong hydrogen bonding (e.g., monensin). Like the crown ethers and cryptands, the ionophores are selective toward the cations with which they will form stable complex compositions.
Structurally, some of the ionophores contain cyclic ether units which appear either as tetrahydrofuran 2,5-diyl or tetrahydropyran 2,6-diyl units. Prominent compounds of this type are monensin and nigericin. These compounds are not polymers and the cyclic ether units contained therein do not appear in segments that provide the array of complexing atoms specified by the present invention.
Additionally, monensin and nigericin are cation complexing agents only when they assume an anionic macrocyclic conformation. Thus, monensin, in the form in which one end group is present as an undissociated carboxylic acid (the other end being hydroxyl), exists in an acyclic conformation and is incapable of forming stable complexes with cations. In its carboxylate anion form, however, hydrogen-bonding between the anion and the distal hydroxyl group causes the molecule to assume the conformation of a macrocycle, the form in which it becomes an effective host for alkali metal cations. (See B. C. Pressman, Annual Revs. in Biochem. 45 pp. 504-507 (1976).)
In addition, certain synthetic compounds have been made that contain multiple tetrahydrofuran 2,5-diyl units. Thus, Y. Kobuke et al, J. Amer. Chem. Soc. 98 pp. 7414-7419 (1976) disclose a homologous series of macrocycles which contain tetrahydrofuran 2,5-diyl units. These compounds have low molecular weight and the tetrahydrofuran 2,5-diyl units are separated by aliphatic linking groups. This separation of the 2,5-diyl units causes a very large decrease in the cation complexing character of these materials.
D. J. Cram et al, Accounts of Chem. Res. 11, pp 8-14 (1978) disclose macrocycles containing multiple tetrahydrofuran 2,5-diyl units. In this case --CH.sub.2 OCH.sub.2 -- units are interspersed between the tetrahydrofuran units. Consequently, these units do not provide the uninterrupted array of heteroatoms of the host polymers useful in this invention.
The polymeric hosts employed in the present invention form stable complexes with an extraordinarily large variety of cations. Moreover, a given polymeric host apparently can adjust its conformation to the coordination requirements of a given cationic guest. Consequently, the host is not specific with respect to the cations with which it will effectively interact. Thus, for example, complexes can be formed with organic cations (e.g., phenyldiazonium; trialkyl and triaryl sulfonium; alkyl ammonium, etc.) Complexes can also be formed with inorganic cations (e.g., ammonium, alkali metal, alkaline earth metal; zinc; thallium; silver; lead; mercury; tin; lanthanum; aluminum; and transition metal including iron, cobalt, molybdenum, chromium, nickel and tungsten.